Add config parameter webassets.load_path

This patch adds support for webassets.load_path configuration option which will be converted to Environment's load_path keyword argument.

This optional argument expects a list of directories to read assets from. To convert the configuration value to a Python list, we pass the value string to ast.literal_eval which converts the string into a proper Python list.

Configuration example:
webassets.load_path = ['path1', 'path2']